Repetition coding for a wireless system

ABSTRACT

A system and method are disclosed for transmitting data over a wireless channel. In some embodiments, transmitting data includes receiving convolutionally encoded data and enhancing the transmission of the data by further repetition encoding the data.

CROSS REFERENCES

The present Application for Patent is a continuation of U.S. patentapplication Ser. No. 13/356,580, entitled “Repetition Coding for aWireless System,” filed Jan. 23, 2012, which is a continuation of U.S.patent application Ser. No. 12/217,300, entitled “Repetition Coding fora Wireless System, filed Jul. 1, 2008, which has been abandoned, whichis a continuation of U.S. patent application Ser. No. 10/666,952,entitled “Repetition Coding for a Wireless System,” filed Sep. 17, 2003,now U.S. Pat. No. 7,418,042, each of which is assigned to the assigneehereof.

FIELD OF THE INVENTION

The present invention relates generally to a data transmission schemefor a wireless communication system. More specifically, a repetitioncoding scheme for a wireless system is disclosed.

BACKGROUND OF THE INVENTION

Wireless communications systems are widely deployed to provide varioustypes of communication content such as voice, video, packet data,messaging, broadcast, and so on. These systems may be multiple-accesssystems capable of supporting communication with multiple users bysharing the available system resources (e.g., time, frequency, andpower). A wireless network, for example a Wireless Local Area Network(WLAN), such as a Wi-Fi network (IEEE 802.11) may include an accesspoint (AP) that may communicate with one or more stations (STAs) ormobile devices. The AP may be coupled to a network, such as theInternet, and enable a mobile device to communicate via the network(and/or communicate with other devices coupled to the access point).

The IEEE 802.11a, 802.11b, and 802.11g standards, which are herebyincorporated by reference, specify wireless communications systems inbands at 2.4 GHz and 5 GHz. The combination of the 802.11a and 802.11gstandards, written as the 802.11a/g standard, will be referred torepeatedly herein for the purpose of example. It should be noted thatthe techniques described are also applicable to the 802.11b standardwhere appropriate. It would be useful if alternate systems could bedeveloped for communication over an extended range or in noisyenvironments. Such communication is collectively referred to herein asextended range communication. The IEEE 802.11a/g standard specifies arobust data encoding scheme that includes error correction. However, forextended range communication, a more robust data transmission scheme atreduced data rates is required.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be readily understood by the followingdetailed description in conjunction with the accompanying drawings,wherein like reference numerals designate like structural elements, andin which:

FIG. 1A shows a diagram illustrating a data portion of a regular802.11a/g OFDM packet, in accordance with various aspects of the presentdisclosure;

FIG. 1B shows a diagram illustrating a data portion of a modified802.11a/g OFDM packet where each symbol is repeated twice (r=2), inaccordance with various aspects of the present disclosure;

FIG. 2A shows a diagram illustrating a transmitter system with arepetition encoder placed after the output of an interleaver such as theone specified in the IEEE 802.11a/g specification, in accordance withvarious aspects of the present disclosure;

FIG. 2B shows a diagram illustrating a receiver system for receiving asignal transmitted by the transmitter system illustrated in FIG. 2A, inaccordance with various aspects of the present disclosure;

FIG. 3A shows a diagram illustrating a transmitter system with arepetition encoder placed before the input of an interleaver designed tohandle repetition coded bits such as the one described below, inaccordance with various aspects of the present disclosure;

FIG. 3B shows a diagram illustrating a receiver system for receiving asignal transmitted by the transmitter system illustrated in FIG. 3A, inaccordance with various aspects of the present disclosure; and

FIGS. 4A-4C show tables illustrating steps of data interleaving by aninterleaver, in accordance with various aspects of the presentdisclosure.

DETAILED DESCRIPTION

It should be appreciated that the present invention can be implementedin numerous ways, including as a process, an apparatus, a system, or acomputer readable medium such as a computer readable storage medium or acomputer network wherein program instructions are sent over optical orelectronic communication links. It should be noted that the order of thesteps of disclosed processes may be altered within the scope of theinvention.

A detailed description of one or more preferred embodiments of theinvention is provided below along with accompanying figures thatillustrate by way of example the principles of the invention. While theinvention is described in connection with such embodiments, it should beunderstood that the invention is not limited to any embodiment. On thecontrary, the scope of the invention is limited only by the appendedclaims and the invention encompasses numerous alternatives,modifications and equivalents. For the purpose of example, numerousspecific details are set forth in the following description in order toprovide a thorough understanding of the present invention. The presentinvention may be practiced according to the claims without some or allof these specific details. For the purpose of clarity, technicalmaterial that is known in the technical fields related to the inventionhas not been described in detail so that the present invention is notunnecessarily obscured.

In a typical system as described below, bits representing a set of datathat is to be communicated are convolutionally encoded or otherwisetransformed into values. Various types of modulation may be used such asBPSK, QPSK, 16 QAM or 32 QAM. In the case of BPSK, which is describedfurther herein, each BPSK symbol may have one of two values and eachBPSK symbol corresponds to one bit. An OFDM symbol includes 48 valuesthat are transmitted on different subchannels. To provide extendedrange, each value that is sent is repeated several times by thetransmitter. In one embodiment, the bits are convolutionally encodedusing the same encoding scheme as the encoding scheme specified for theIEEE 802.11a/g standard. Each encoded value is repeated and transmitted.Preferably, the values are repeated in the frequency domain, but thevalues may also be repeated in the time domain. In some embodiments, therepetition coding is implemented before interleaving and a speciallydesigned interleaver is used to handle repeated values. In addition, apseudorandom code may be superimposed on OFDM symbols to lower thepeak-to-average ratio (PAR) of the transmitted signal.

The receiver combines each of the signals that correspond to repetitioncoded values and then uses the combined signal to recover the values. Inembodiments where the values are combined in the frequency domain, thesignals are combined coherently with correction made for different subchannel transfer functions and phase shift errors. For the purpose ofthis description and the claims, “coherently” combining should not beinterpreted to mean that the signals are perfectly coherently combined,but only that some phase correction is implemented. The signals fromdifferent subchannels are weighted according to the quality of eachsubchannel. A combined subchannel weighting is provided to a Viterbidetector to facilitate the determination of the most likely transmittedsequence.

Using the modulation and encoding scheme incorporated in the IEEE802.11a/g standard, a required signal to noise ratio (SNR) decreaseslinearly with data rate assuming the same modulation technique and basecode rate are not changed and repetition coding is used. Some furthergains could be achieved through the use of a better code or outer code.However, in a dual mode system that is capable of implementing both theIEEE 802.11a/g standard and an extended range mode, a complexityintroduced by those techniques may not be worth the limited gains thatcould be achieved. Implementing repetition of values is, in comparison,simpler and more efficient in many cases.

The repetition code can be implemented either in the time domain or inthe frequency domain. For time domain repetition, OFDM symbols in thetime domain (after the Inverse Fast Fourier Transform (IFFT) operation)are repeated a desired number of times, depending on a data rate. Thisscheme has an advantage in efficiency since just one guard interval isrequired for r-repeated OFDM symbols in the time domain.

FIG. 1A shows a diagram illustrating a data portion of a regular802.11a/g OFDM packet, in accordance of various aspects of the presentdisclosure. Each OFDM symbol 104 is separated by a guard band 102. FIG.1B shows a diagram illustrating the data portion of a modified 802.11a/gOFDM packet where each symbol is repeated twice (r=2). Each set ofrepeated symbols 114 is separated by a single guard band 112. There isno need for a guard band between the repeated symbols.

The OFDM symbols can also be repeated in the frequency domain (beforethe IFFT). The disadvantage of this scheme is that one guard intervalhas to be inserted between every OFDM symbol in the time domain sincethe OFDM symbols with frequency-domain repetition are not periodic.However, repetition in the frequency domain can achieve better multipathperformance if the repetition pattern is configured in the frequencydomain to achieve frequency diversity.

In a typical environment where signals are reflected one or more timesbetween a transmitter and a receiver, it is possible that certainreflections and direct signals will tend to cancel out at the receiverbecause a phase difference between the paths could be close to 180degrees. For different frequencies, the phase difference between thepaths will be different and so spreading the repeated values amongdifferent frequencies to achieve frequency diversity ensures that atleast some of the values will arrive at the receiver with sufficientsignal strength to be combined and read. To maximize the benefit offrequency diversity, it is preferable to repeat values acrosssubchannels that are as widely spaced as is practicable, since the phasedifference between adjacent subchannels is small.

FIG. 2A shows a diagram illustrating a transmitter system with arepetition encoder placed after the output of an interleaver such as theone specified in the IEEE 802.11a/g specification, in accordance withvarious aspects of the present disclosure. In this example system, BPSKmodulation is implemented and the repetition encoder and the interleaverare described as operating on bits, which is equivalent to operating onthe corresponding values. In other embodiments, other modulation schemesmay be used and values may be repeated and interleaved. The interleaveris included in the IEEE 802.11a/g transmitter specification for thepurpose of changing the order of the bits sent to remove correlationamong consecutive bits introduced by convolutional encoder 202. Incomingdata is convolutionally encoded by the convolutional encoder 202. Theoutput of convolutional encoder 202 is interleaved by IEEE 802.11a/ginterleaver 204. Repetition encoder 206 repeats the bits andpseudorandom mask combiner 208 combines the output of repetition encoder206 with a pseudorandom mask for the purpose of reducing the PAR of asignal, as is described below. The signal is then processed by IFFTprocessor 210 before being transmitted.

FIG. 2B shows a diagram illustrating a receiver system for receiving asignal transmitted by the transmitter system illustrated in FIG. 2A, inaccordance with various aspects of the present disclosure. The receivedsignal is processed by FFT processor 220. The output of FFT processor220 is input to mask remover 218 which removes the pseudorandom mask.Data combiner 216 combines the repetition encoded data into a stream ofnonrepetitive data. The operation of data combiner 216 is described infurther detail below. IEEE 802.11a/g deinterleaver 214 deinterleaves thedata and Viterbi decoder 212 determines the most likely sequence of datathat was input to the transmission system originally.

The transmitter and the receiver systems illustrated in FIGS. 2A and 2Bcan use the same interleaver and deinterleaver as the regular 802.11 a/gsystem, and also has flexibility in designing the repetition patternsince the repetition coder 206 is placed right before the IFFT block210. However, it has certain disadvantages. Data padding is required atthe transmitter and data buffering is required at the receiver. Bitshave to be padded according to a number of bytes to be sent and a datarate. The number of padded bits is determined by how many bits one OFDMsymbol can carry. Since a 802.11a/g interleaver works with 48 coded bitsfor BPSK modulation, bits need to be padded to make the number of codedbits a multiple of 48. Since the repetition coder 206 is placed afterthe interleaver 204, it may be necessary to pad the data by addingunnecessary bits for lower data rates than 6 Mbps.

For example, one OFDM symbol would carry exactly 1 uncoded repeated bitat a data rate of ¼ Mbps. Since the OFDM symbol could be generated fromthat one bit, there would never be a need to add extra uncoded bits andso padding would not be necessary in principle. However, due to thespecial structure of the 802.11a/g interleaver, several bits would needto be padded to make the number of coded bits a multiple of 48 beforethe interleaver. The padded bits convey no information and add to theoverhead of the transmission, making it more inefficient.

On the other hand, if the repetition encoder is placed after theinterleaver, the repetition coded bits generated from the 48 interleavedbits are distributed over multiple OFDM symbols. Therefore, a receiverwould need to process the multiple OFDM symbols before deinterleavingthe data could be performed. Therefore, additional buffers would benecessary to store frequency-domain data.

The system can be improved and the need for data padding at thetransmitter and data buffering at the receiver can be eliminated byredesigning the interleaver so that it operates on bits output from therepetition encoder.

FIG. 3A shows a diagram illustrating a transmitter system with arepetition encoder 304 placed before the input of an interleaver 306designed to handle repetition coded bits such as the one describedbelow, in accordance with various aspects of the present disclosure.Incoming data is convolutionally encoded by convolutional encoder 302.The output of convolutional encoder 302 is repetition coded byrepetition encoder 304. The interleaver 306 interleaves the repetitioncoded bits. The interleaver 306 is designed so that data padding is notrequired and so that for lower repetition levels, bits are interleavedso as to separate repeated bits. Pseudorandom mask combiner 308 combinesthe output of the interleaver 306 with a pseudorandom mask for thepurpose of reducing PAR of a signal, as is described below. The signalis then processed by IFFT processor 310 before being transmitted.

FIG. 3B shows a diagram illustrating a receiver system for receiving asignal transmitted by the transmitter system illustrated in FIG. 3A, inaccordance with various aspects of the present disclosure. The receivedsignal is processed by FFT processor 320. The output of the FFTprocessor 320 is input to mask remover 318 which removes thepseudorandom mask. Deinterleaver 316 deinterleaves the data. Datacombiner 314 combines the repetition encoded data into a stream ofnonrepetitive data. The operation of data combiner 314 is described infurther detail below. Viterbi decoder 312 determines the most likelysequence of data that was input to the transmission system originally.

Interleaver 306 is preferably designed such that the same (repeated)data are transmitted well separated in the frequency domain to achievefull frequency diversity. For example, a repetition pattern in thefrequency domain for in 1 Mbps mode in one embodiment would repeat eachbit 6 times. Denoting data in the frequency domain as d₁, d₂, . . . ,d_(g), the repeated sequence of data is given by:d₁d₁d₁d₁d₁d₁d₂d₂d₂d₂d₂d₂ . . . d₈d₈d₈d₈d₈d₈

The same data are placed in a group fashion because it is easy tocombine those data at a receiver. Note that the repeated data can becombined only after r (6 in this example) data are available.

The repetition pattern in the above example does not provide thegreatest possible frequency diversity since the spacing between the samedata transmitted on adjacent subchannels may not be large enough and thesubchannels corresponding to the same data are not completelyindependent. Greater frequency diversity would be desirable especiallyfor multipath channels with large delay spreads. Interleaver 306,therefore, is designed to spread the repeated data in the frequencydomain to achieve frequency diversity as much as is practical.

FIGS. 4A-4C show tables illustrating steps of data interleaving by aninterleaver, in accordance with various aspects of the presentdisclosure. In one embodiment, an interleaver is designed to optimizethe frequency diversity provided by the interleaver for data ratesfaster than 1 Mbps (repetition number <=6). For lower data rates ½ and ¼Mbps, there is enough repetition that sufficient subchannels are coveredto provide frequency diversity even if adjacent subchannels are used. Inthe preferred interleaver described below, repeated bits are separatedat least by 8 subchannels and consecutive coded bits from theconvolutional encoder are separated at least by 3 subchannels. Theinterleaver is designed according to the following steps:

-   -   1. A 6×8 table is generated as shown in FIG. 4A to satisfy the        first rule which specifies that bits are separated at least by 8        subchannels.    -   2. As shown in FIG. 4B, the columns are swapped to meet the        second rule which specifies that consecutive coded bits are        separated at least by 3 subchannels.    -   3. As shown in FIG. 4C, separation between repeated bits is        increased by swapping rows. In the example shown, repeated bits        are separated by at least 16 bins for 3 Mbps (Repetition        number=2 for 3 Mbps so each bit is repeated once.)

For the example interleaver shown, if the input to the interleaver is{1, 2, 3, . . . , 48}, then the output would be: {1, 19, 37, 7, 25, 43,13, 31, 4, 22, 40, 10, 28, 46, 16, 34, 2, 20, 38, 8, 26, 44, 14, 32, 5,23, 41, 11, 29, 47, 17, 35, 3, 21, 39, 9, 27, 45, 15, 33, 6, 24, 42, 12,30, 48, 18, 36}.

Repetition of the values in the frequency domain tends to generate apeak in the time domain, especially for very low data rates (i.e., forlarge repetition numbers). The large PAR causes problems for the system,especially the transmit power amplifier. This problem can be amelioratedby scrambling or masking the values transmitted on different frequenciesso that they are not all the same. As long as the masking scheme isknown, the scrambling can be undone at the receiver. In one embodiment,the frequency-domain data is multiplied by the long symbol of 802.11a/g,which was carefully designed in terms of PAR. As can be seen in FIG. 2,a mask operation is performed right before the IFFT operation. Ingeneral, any masking sequence can be used that causes repeated values todiffer enough that the PAR is suitably reduced. For example, apseudorandom code is used in some embodiments.

At the receiver, decoding includes: (1) mask removal, (2)deinterleaving, (3) data combining, (4) channel correction, (5) Viterbidecoding. It should be noted that in some embodiments, the order of thesteps may be changed as is appropriate.

In embodiments using frequency repetition, the transmitter preferablymasks the frequency-domain signal to reduce the PAR in the time-domain.The receiver removes the mask imposed by the transmitter. If, as in theexample above, the mask used by the transmitter consists of +/−1 s, thenthe mask is removed by changing the signs of the FFT outputs in thereceiver. After the mask is removed, the data is deinterleaved accordingto the interleaving pattern at the transmitter.

The repeated signal is combined in the frequency domain at the receiverto increase an SNR of the repeated signal over an SNR had the signal notbeen repeated. The SNR is increased by multiplying the complex conjugateof the channel response as follows.

$Y_{c} = {\sum\limits_{j \in S_{c}}{H_{j}^{*}Y_{j}}}$$H_{c} = {\sum\limits_{j \in S_{c}}{H_{j}}^{2}}$where Y_(j) is the signal in subchannel j, H_(j) is the response ofsubchannel j, Y_(c) is the combined signal, H_(c) is the combinedchannel, and S_(c) is the set of indices corresponding to the frequencysubchannels that contain the same data.

The channel effect is preferably removed before the data is input to theViterbi decoder so that the Viterbi decoder is able to use the same softdecision unit regardless of the actual channel response. In theextended-range mode, the combined channel is used in the channelcorrection unit.

The frequency-domain signals are weighted for calculating the pathmetrics in the soft-decision Viterbi decoder, and the optimal weightsare determined by the corresponding SNR.

The resulting SNR for the combined signal becomes:

${S\; N\; R} = {\sum\limits_{j \in S_{c}}{{H_{j}}^{2}\frac{E_{x}}{\sigma_{j}^{2}}}}$where E_(x) is the signal power, and σ_(j) ² is the noise power for thesubchannel j. The combinedSNR is used to evaluate the Viterbi weights.

The 802.11a/g standard specifies that there are four pilot signalsincluded in each OFDM symbol for the purpose of estimating timing offsetand frequency offset and tracking phase noise in 802.11a/g signals. The802.11a/g system assumes that these 4 pilots are reliable enough toestimate the phase information. That assumption may not be true for asystem with a very low SNR. The redundancy that exists in thefrequency-domain signal is exploited to help the pilots to estimate andtrack phase.

The phase information is estimated from the frequency domain data asfollows:

1. The repeated signals are combined in the frequency domain to increasean SNR, with a channel estimate determined from a preamble sequence oflong symbols and an estimated slope, which captures the effect of timingoffset.

2. Hard decisions are made for each of the combined signals afterremoving the phase offset estimated from the previous symbol.

3. The combined signals are multiplied by their own hard decisions. Theaverage of the hard-decision corrected signal is used to evaluate anangle to estimate the phase offset for the current symbol.

A filter is applied to the estimated phase offset to reduce the effectof noise. In one embodiment, a nonlinear median filter is used. Thenonlinear median filter effectively detects and corrects an abruptchange in the phase offset, which could be caused by hard decisionerrors.

An encoding and decoding scheme for a wireless system has beendisclosed. Preferably, repetition coding in the frequency domain isused. An interleaver that provides frequency diversity has beendescribed. In various embodiments, the described techniques may becombined or used separately according to specific system requirements.

Although the foregoing invention has been described in some detail forpurposes of clarity of understanding, it will be apparent that certainchanges and modifications may be practiced within the scope of theappended claims. It should be noted that there are many alternative waysof implementing both the process and apparatus of the present invention.Accordingly, the present embodiments are to be considered asillustrative and not restrictive, and the invention is not to be limitedto the details given herein, but may be modified within the scope andequivalents of the appended claims.

What is claimed is:
 1. A method for transmitting data in a wirelesscommunication system, comprising: encoding data using a modulation andcoding scheme; repeating the encoded data using a repetition pattern togenerate repetition-encoded data, wherein the repetition pattern isbased at least in part on a data rate; interleaving therepetition-encoded data according to a rule, the rule comprising a firstrule and a second rule, wherein the first rule specifies that adjacentbits of the repetition-encoded data are separated by a first number ofsubchannels and the second rule specifies that consecutive coded bits ofthe repetition-encoded data are separated by a second number ofsubchannels, and wherein the first and the second numbers of subchannelsare finite positive integers and the first number of subchannels isdifferent from the second number of subchannels; and transmitting theinterleaved data over a channel.
 2. The method of claim 1, whereinrepeating the encoded data using the repetition pattern comprisesduplicating the encoded data and wherein transmitting the interleaveddata over the channel comprises transmitting the duplicated data over anumber of subchannels of the channel.
 3. The method of claim 1, whereinthe modulation and coding scheme comprises binary phase shift keying(BPSK) modulation and a code rate of 1/2.
 4. The method of claim 1,wherein the data is encoded using a modulation scheme from the groupconsisting of quadrature phase shift keying (QPSK), 16-quadratureamplitude modulation (16-QAM), and 32-quadrature amplitude modulation(32-QAM).
 5. The method of claim 1, wherein the data comprises anorthogonal frequency division multiplexing (OFDM) symbol including apredetermined number of bits, wherein each bit is mapped to andtransmitted on a different subchannel of the channel.
 6. The method ofclaim 1, wherein the data is repeated in one of a frequency domain and atime domain.
 7. The method of claim 1, further comprising: inserting anumber of pilot signals in the data transmitted over the channel.
 8. Themethod of claim 1, further comprising: converting the data into a timedomain using an inverse fast Fourier transform.
 9. An apparatus fortransmitting data in a wireless communication system, comprising: anencoder to encode data using a modulation and coding scheme; arepetition encoder to repeat the encoded data using a repetition patternto generate repetition-encoded data, wherein the repetition pattern isbased at least in part on a data rate; an interleaver to interleave therepetition-encoded data according to a rule, the rule comprising a firstrule and a second rule, wherein the first rule specifies that adjacentbits of the repetition-encoded data are separated by a first number ofsubchannels and the second rule specifies that consecutive coded bits ofthe repetition-encoded data are separated by a second number ofsubchannels, and wherein the first and the second numbers of subchannelsare finite positive integers and the first number of subchannels isdifferent from the second number of subchannels; and a transmitter totransmit the interleaved data over a channel.
 10. The apparatus of claim9, wherein repeating the encoded data using the repetition patterncomprises duplicating the encoded data and wherein transmitting theinterleaved data over the channel comprises transmitting the duplicateddata over a number of subchannels of the channel.
 11. The apparatus ofclaim 9, wherein the modulation and coding scheme comprises binary phaseshift keying (BPSK) modulation and a code rate of 1/2.
 12. The apparatusof claim 9, wherein the data is encoded using a modulation scheme fromthe group consisting of quadrature phase shift keying (QPSK),16-quadrature amplitude modulation (16-QAM), and 32-quadrature amplitudemodulation (32-QAM).
 13. The apparatus of claim 9, wherein the datacomprises an orthogonal frequency division multiplexing (OFDM) symbolincluding a predetermined number of bits, wherein each bit is mapped toand transmitted on a different subchannel of the channel.
 14. Theapparatus of claim 9, wherein the data is repeated in one of a frequencydomain and a time domain.
 15. The apparatus of claim 9, furthercomprising: a processor to convert the data into a time domain using aninverse fast Fourier transform.
 16. A non-transitory computer-readablemedium storing instructions executable by a processor to cause a deviceincluding the non-transitory computer-readable medium and the processorto: encode data using a modulation and coding scheme; repeat the encodeddata using a repetition pattern to generate repetition-encoded data,wherein the repetition pattern is based at least in part on a data rate;interleave the repetition-encoded data according to a rule, the rulecomprising a first rule and a second rule, wherein the first rulespecifies that adjacent bits of the repetition-encoded data areseparated by a first number of subchannels and the second rule specifiesthat consecutive coded bits of the repetition-encoded data are separatedby a second number of subchannels, and wherein the first and the secondnumbers of subchannels are finite positive integers and the first numberof subchannels is different from the second number of subchannels; andtransmit the interleaved data over a channel.
 17. The non-transitorycomputer-readable medium of claim 16, wherein repeating the encoded datausing the repetition pattern comprises duplicating the encoded data andwherein transmitting the interleaved data over the channel comprisestransmitting the duplicated data over a number of subchannels of thechannel.
 18. The non-transitory computer-readable medium of claim 16,wherein the modulation and coding scheme comprises binary phase shiftkeying (BPSK) modulation and a code rate of 1/2.
 19. The non-transitorycomputer-readable medium of claim 16, wherein the data comprises anorthogonal frequency division multiplexing modulation (OFDM) symbolincluding a predetermined number of bits, wherein each bit is mapped toand transmitted on a different subchannel of the channel.
 20. Thenon-transitory computer-readable medium of claim 16, wherein the data isrepeated in one of a frequency domain and a time domain.